Supporting non-standard pointer types in STL containers

[Bob Walters]

> a quick feedback: any possible change in this area will go in  
> mainline only. Thus, in order to allow the maintainers to further  
> investigate this work, please provide the changes to stl_tree.h in  
> in the form of a diff vs current mainline.

I'm posting a patch for stl_tree.h which shows the code changes needed  
to support alternative pointer types.   I've confirmed using the  
testsuite that this set of changes created no new errors when run with  
normal pointer types.   The changes in this case included some work to  
preserve ABI compatibility with code compiled in previous versions of  
libstdc++-v3.

To test this with an alternative pointer implementation, I wrote and  
used a relative_ptr<>, to confirm a case where a non-standard pointer  
could work.  My testing with this pointer hasn't been as exhaustive as  
the regular testsuite.  This alternative pointer doesn't need to  
become part of libstdc++, but could be added to ext/ or to the test  
suite as an example.

A pdf on google docs explains the design, and will be shared out to  
this list.

Again, I appreciate any time spent looking this over, and any  
suggestions, bug reports, etc.
Thanks,

- Bob Walters

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